1. Field of the Invention
This invention relates to a rotary actuator for a disk drive used as external storage in a computer system. More particularly, this invention relates to a rotary actuator for positioning a read/write head onto the target track in a disk drive. This invention is intended to provide a rotary actuator having an increased access speed and high reliability.
2. Description of the Related Art
A magnetic disk drive or an optical disk drive has made a remarkable progress in storage capacity in recent years. For example, a storage capacity of 1 giga-bytes per spindle can be obtained in a hard magnetic disk drive.
There are two types in positioning the magnetic heads: (1) linear actuator type and (2) rotary actuator type. This invention relates to the latter type and an example of the prior art technology is disclosed in Japanese Unexamined Patent Publication SHO-56-88663 by I. Kitamura et al. dated on July 18, 1981.
FIGS. 1 and 2 show schematically a fundamental structure of the rotary actuator and a disk assembly DA, and a disk drive respectively. FIG. 1 shows a perspective view, in which the stacked magnetic disk assembly DA and the rotary actuator assembly OM with magnetic heads are separated. FIG. 2 shows a plan view of the disk drive, in which these component units are assembled.
A spindle 36 having a plurality of disks 37 at equal intervals (in FIG. 1 10 disks are shown) rotates at a speed of 3600 rpm. The data are recorded on both surfaces of the disk 37, and a magnetic head 31, two magnetic heads being arranged on each arm 32, is positioned on a specified track on the disk surface by an actuator movement and perform a read/write operation.
In the rotary actuator type, the arm 32 of the actuator assembly can move or rotate within a specified angle .theta. as shown in FIG. 2, and the movement thereof is precisely controlled by a rotary actuator OM. The rotary actuator OM can not rotate continuously but rotate within a limited angle range, therefore, the motor OM is hereinafter called a rotary actuator in the present invention.
The rotary actuator OM includes a stator assembly 34 and a rotor assembly 33. The stator assembly 34 comprises a stator housing 39 and a yoke and coils (these are not shown). The rotor assembly 33 comprises a plurality of arms 32 fixed to a rotor yoke (not shown) in a projecting way. A number of arms is equal to that of disks or in a range between number of disks plus one and minus one (9 arms are shown in FIG. 1). At each far end of arms 32, a head assembly 30 is disposed having two magnetic heads 31.
As shown in FIG. 2, the rotary actuator OM is arranged at a corner of a disk enclosure 38, and the stator housing 39 forms a part of a completed disk enclosure. The rotary actuator OM is positioned in such a way that, when the rotor 33 rotates within the angle .theta., the magnetic head 31 located at far end of the combination of arm 32 and head assembly 30, can sweep the entire recording region on the disk surface.
In order to reduce an access time in the disk drive, it is desirable that the rotary actuator OM has a high torque performance.
As a structure of the rotary actuator, two types have been known and utilized; (1) moving coil type and (2) moving magnet type. The moving coil type comprises a stator with permanent magnets and a rotor with coils, and the moving magnet type comprises a stator with coils and a rotor with permanent magnets. An example of the latter type is disclosed in U.S. Pat. No. 4,346,416, issued on Aug. 24, 1982 to C. M. Riggle etal., and the same is also disclosed in Japanese Patent Publication SHO-62-35181 dated on July 31, 1987.
FIG. 3 shows a schematic cross section of the rotary actuator OM of the prior art, the cross section being vertical to the longitudinal axis thereof. A stator includes a yoke 61 and a flat coil 64 arranged on an inside surface of the yoke 61 and further includes an axis 63 fixed to the stator. The coil 64 has a shape as shown in FIG. 4. A rotor 62 can move around the axis 63 and holds two permanent magnets 65 and 66, which are elongated in a direction vertical to the sheet and facing parallel windings 64a and 64b in a longitudinal direction. The polarity arrangements of two permanent magnets 65 and 66 are reversed. The permanent magnet 65 has an S pole facing the coil winding 64a and the permanent magnet 66 has an N pole facing the other coil winding 64b.
When the rotary actuator OM is actuated, an electrical current flows along the coil winding. The directions of the current flow along the coil windings 64a and 64b are opposite and further the directions of magnetic flux in the gap regions are also opposite, therefore, the rotor is subjected to two torques working additively. The movement of the rotor 62 can be precisely controlled by the direction and the magnitude of the electrical current.
The structure of the rotary actuator OM shown in FIG. 3 develops a comparatively small torque because it has only one pair of poles (two pole type). In order to obtain a larger torque, a structure having two pair of poles (four pole type) and two coils, is disclosed in Japanese Unexamined Patent Publication SHO-60-5480 dated on Jan. 12, 1985.
The four pole type of a rotary actuator OM is schematically shown in FIG. 5. Two flat coils 74 are arranged on an inside surface of a yoke 71, and four permanent magnets 75 to 78 are fixed on a rotor 72. This type of the rotary actuator OM has a capacity to develop a larger torque and an effect to reduce the magnitude of the magnetic flux compared with the two pole type, resulting in making a thickness of the coil thin.
In designing a rotary actuator having a rotor with four magnetic poles, it becomes a problem to arrange plural permanent magnets in a limited area of the rotor surface. In FIG. 5, the details of a mechanical structure for supporting a plurality of arms are not shown for simplicity, each arm having a head assembly. The arm assembly restricts utilization of the entire surface of the rotor, and further, the radius of the rotor is smaller than that of the yoke. Therefore, a circumferential width of each permanent magnet is inevitably reduced. This fact reduces the intensity of the magnetic flux in the gap between the permanent magnet and the coil, resulting in reducing the torque of the rotary actuator.